1. Technical Field
The present disclosure relates generally to making measurements of subsurface formations surrounding a wellbore using a bottomhole assembly (BHA) that is integral to a drill string, and particularly to the allocation of electrical power among the downhole tools comprising the BHA.
2. Background Art
Logging tools have long been used in wellbores to make, for example, formation evaluation measurements to infer properties of the formations surrounding the borehole and the fluids in the formations. Common logging tools include electromagnetic tools, nuclear tools, and nuclear magnetic resonance (NMR) tools, though various other tool types are also used.
Early logging tools were run into a wellbore on a wireline cable, after the wellbore had been drilled. Modern versions of such wireline tools are still used extensively. However, the need for information while drilling the borehole gave rise to measurement-while-drilling (MWD) tools and logging-while-drilling (LWD) tools. MWD tools typically provide drilling parameter information such as weight on the bit, torque, temperature, pressure, direction, and inclination. LWD tools typically provide formation evaluation measurements such as resistivity, porosity, and NMR distributions (e.g., T1 and T2). MWD and LWD tools often have components common to wireline tools (e.g., transmitting and receiving antennas), but MWD and LWD tools must be constructed to not only endure but to operate in the harsh environment of drilling.
A BHA typically includes a single MWD tool and several LWD tools that are connected by a low power transmission wire or bus called the “LTB”. The LTB provides power to the logging tools and also provides a communication link by which the tools can communicate with one another. The source of this power is a turbine generator in the MWD tool that is driven by pressurized drilling fluid (“mud”) when mud pumps are on. The MWD turbine generator, however, can only provide a limited amount of power. Thus, this restricts the possible configurations of a BHA, or at least limits the number of tools in a BHA that can be operated simultaneously.